The present invention relates to a method for producing a flexible elastomeric polyurethane skin wherein a polyurethane reaction mixture is sprayed onto a mould surface and the polyurethane reaction mixture is allowed to cure to produce the skin layer. The polyurethane reaction mixture is composed of components comprising at least an isocyanate component, isocyanate-reactive components and a catalyst component, the isocyanate component being composed of at least one isocyanate having at least two NCO-groups which are not directly attached to an aromatic group, and the catalyst component being substantially free of lead.
Polyurethane skins are used mainly in interior trim parts of automotive vehicles, more particularly in instrument panels, door panels, consoles, glove compartment covers, etc. In such applications the polyurethane skin is adhered to a rigid substrate by means of an intermediate semi-rigid backfoam layer, which is situated between the elastomeric skin and the substrate. This backfoam layer has a density which is lower than 200 kg/m3 and which is usually comprised between 120 and 180 kg/m3. The presence of such backfoam layer enables to indent the polyurethane skin resiliently so that a soft touch is provided to the trim part. An essential feature of the elastomeric polyurethane skin is in this respect that it has to be sufficiently flexible, i.e. it should have a flexural modulus which is at least smaller than 30 MPa.
Suitable polyurethane reaction mixtures for spraying flexible elastomeric polyurethane skins are disclosed for example in EP-B-0 379 246. The compositions disclosed in this patent are based on aliphatic isocyanates and result in light-stable polyurethanes which do not require an in-mould coating (or a post-painting step) to avoid discoloration of the skin. The Colo-Fast® aliphatic polyurethane formulations (composed of a Polyfast® and an Isofast® blend: trademarks of Recticel) which were produced according to the teachings of this patent, enabled to achieve flexible polyurethane skins having a flexural modulus, measured in accordance with ASTM D790-03, of between 20 and 30 MPa. These formulations further had a relatively short cure time which was typically less than 180 seconds, i.e. the skins produced with these formulations had built up a sufficient green strength within such a cure time so that they could be removed from the mould surface without producing remaining deformations of the skin.
A problem with these aliphatic polyurethane formulations is however that the organolead catalysts which were used therein are or will be forbidden in the future because of environmental regulations. Alternative formulations are now available wherein the organolead catalyst is replaced for example by a combination of an organobismuth and an organotin catalyst, and optionally further in combination with an organozinc catalyst (see for example WO 2004/000905). Although such catalyst combinations also allow to achieve cure times of 180 seconds or less, they produce a somewhat different polyurethane network resulting in stiffer polyurethane skins, more particularly in polyurethane skins having a flexural modulus of about 40 MPa or even higher.
A further drawback of replacing the organolead catalyst by other organometal catalysts is that also the emission of volatile organic compounds from the polyurethane elastomer is increased thereby. As disclosed in WO 2004/000905, the emission can be reduced by using special organobismuth or organotin catalysts wherein the metal atom is bonded to long chain organic groups such as oleyl, linoleyl or linolenyl groups. In practice however the use of these catalysts may give processing problems due to a lower compatibility in the polyol blend wherein they are added. The emission of volatile organic compounds can further be reduced by increasing the NCO-index which results however in stiffer polyurethane elastomers, having for example a flexural modulus of about 55 MPa.
Instead of producing the polyurethane skins from aliphatic polyurethane formulations, it is also possible to produce them from aromatic polyurethane formulations, i.e. from polyurethane formulations the isocyanate component of which comprises an aromatic instead of an aliphatic polyisocyanate. As disclosed for example in EP-B-1 079 962 such aromatic polyurethane formulations have important advantages over aliphatic polyurethane formulations. Aromatic polyurethane formulations produce polyurethane elastomers having more particularly better physical properties such as higher tensile and tear strengths and better elongation and “cold flex” capability. They are also cheaper and have faster cure rates and consequently shorter demoulding times than aliphatic polyurethane formulations. Consequently, they do not require an organolead catalyst for achieving a short demoulding time. Moreover, they do not release volatile organic compounds (VOCs) or at least considerably less than aliphatic polyurethanes.
A drawback of aromatic polyurethane elastomers is however that they become less stable after prolonged exposure to light so that they have to be masked from direct exposure to sunlight by means of an outer coating layer. This coating layer is preferably an in-mould coating layer which is applied onto the mould surface before spraying the aromatic polyurethane formulation thereon. The coating layer is either solvent or water based and has a thickness of less than about 40 microns. Solvent based in-mould coatings have the drawback of releasing volatile organic compounds (VOCs) whilst water based in-mould coatings require considerably longer dry times even when using a heat source as disclosed in EP-B-1 079 962.
Instead of making the polyurethane skin of one polyurethane layer, it is also known from different patent applications, in particular from US 2006/0008622 and US 2006/0091574, to make composite polyurethane skins comprising an outer polyurethane layer, having a density higher than 850 kg/m3, and an expanded polyurethane layer, having a density of between 100 and 750 kg/m3. The expanded polyurethane layer comprises an aromatic polyurethane elastomer whilst the outer polyurethane layer may either be made of an aromatic or of an aliphatic polyurethane elastomer. When the outer polyurethane layer is made of an aromatic polyurethane elastomer, it still has to be provided with a coating which inhibits sunlight and/or other ultraviolet light from reaching the outer polyurethane layer. Although these prior art documents do not mention any flexibility of the composite polyurethane skins, it will be clear that when a highly flexible skin is to be produced, in particular a skin having a flexural modulus smaller than 30 MPa or even still smaller, a skilled person will make the composite skins disclosed in these US patent applications entirely of aromatic polyurethane elastomers.